A screwdriver bit structure having auxiliary positioning function includes a screwdriver bit of which one end is a circular cylinder and having a driving portion that is any one of a slotted type, a cross type and a polygon type. Another end of the circular cylinder is a fitting portion that is buckled to the screwdriver bar. The features of the invention are that the circular cylinder has a concave ring groove near the front of the driving portion. An auxiliary sheath is a long tube body that has a tube hole. The internal diameter of the tube hole is slightly larger than the periphery of the circular cylinder. A protruding ring is disposed to an internal wall of the tube hole. Accordingly, the tube hole of the auxiliary sheath can cover the periphery of the circular cylinder to allow the screwdriver bit to achieve the absolute positioning.